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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Behavior Engineering for Hindrance I. van Beijnum 3 Avoidance IMDEA Networks 4 Internet-Draft November 4, 2010 5 Intended status: Standards Track 6 Expires: May 8, 2011 8 An FTP ALG for IPv6-to-IPv4 translation 9 draft-ietf-behave-ftp64-06 11 Abstract 13 The File Transfer Protocol (FTP) has a very long history, and despite 14 the fact that today, other options exist to perform file transfers, 15 FTP is still in common use. As such, it is important that in the 16 situation where some client computers only have IPv6 connectivity 17 while many servers are still IPv4-only and IPv6-to-IPv4 translators 18 are used to bridge that gap, FTP is made to work through these 19 translators as best it can. 21 FTP has an active and a passive mode, both as original commands that 22 are IPv4-specific, and as extended, IP version agnostic commands. 23 The only FTP mode that works without changes through an IPv6-to-IPv4 24 translator is extended passive. However, many existing FTP servers 25 do not support this mode, and some clients do not ask for it. This 26 document specifies a middlebox that may solve this mismatch. 28 Status of this Memo 30 This Internet-Draft is submitted in full conformance with the 31 provisions of BCP 78 and BCP 79. 33 Internet-Drafts are working documents of the Internet Engineering 34 Task Force (IETF). Note that other groups may also distribute 35 working documents as Internet-Drafts. The list of current Internet- 36 Drafts is at http://datatracker.ietf.org/drafts/current/. 38 Internet-Drafts are draft documents valid for a maximum of six months 39 and may be updated, replaced, or obsoleted by other documents at any 40 time. It is inappropriate to use Internet-Drafts as reference 41 material or to cite them other than as "work in progress." 43 This Internet-Draft will expire on May 8, 2011. 45 Copyright Notice 47 Copyright (c) 2010 IETF Trust and the persons identified as the 48 document authors. All rights reserved. 50 This document is subject to BCP 78 and the IETF Trust's Legal 51 Provisions Relating to IETF Documents 52 (http://trustee.ietf.org/license-info) in effect on the date of 53 publication of this document. Please review these documents 54 carefully, as they describe your rights and restrictions with respect 55 to this document. Code Components extracted from this document must 56 include Simplified BSD License text as described in Section 4.e of 57 the Trust Legal Provisions and are provided without warranty as 58 described in the Simplified BSD License. 60 Table of Contents 62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 63 2. Notational Conventions . . . . . . . . . . . . . . . . . . . . 4 64 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 65 4. ALG overview . . . . . . . . . . . . . . . . . . . . . . . . . 4 66 5. Control channel translation . . . . . . . . . . . . . . . . . 5 67 6. EPSV to PASV translation . . . . . . . . . . . . . . . . . . . 7 68 7. EPRT to PORT translation . . . . . . . . . . . . . . . . . . . 8 69 7.1. Stateless EPRT translation . . . . . . . . . . . . . . . . 8 70 7.2. Stateful EPRT translation . . . . . . . . . . . . . . . . 9 71 8. Default port 20 translation . . . . . . . . . . . . . . . . . 10 72 9. Both PORT and PASV . . . . . . . . . . . . . . . . . . . . . . 10 73 10. Default behavior . . . . . . . . . . . . . . . . . . . . . . . 10 74 11. The ALGS command . . . . . . . . . . . . . . . . . . . . . . . 11 75 12. Timeouts and translating to NOOP . . . . . . . . . . . . . . . 12 76 13. IANA considerations . . . . . . . . . . . . . . . . . . . . . 13 77 14. Security considerations . . . . . . . . . . . . . . . . . . . 13 78 15. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 14 79 16. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14 80 17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 81 17.1. Normative References . . . . . . . . . . . . . . . . . . . 14 82 17.2. Informative References . . . . . . . . . . . . . . . . . . 15 83 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 15 85 1. Introduction 87 [RFC0959] specifies two modes of operation for FTP: active mode, in 88 which the server connects back to the client and passive mode, where 89 the server opens a port for the client to connect to. Without 90 additional measures, active mode with a client-supplied port does not 91 work through NATs or firewalls. With active mode, the PORT command 92 has an IPv4 address as its argument, and in passive mode, the server 93 responds to the PASV command with an IPv4 address. This makes both 94 the passive and active modes as originally specified in [RFC0959] 95 incompatible with IPv6. These issues were solved in [RFC2428], which 96 introduces the EPSV (extended passive) command, where the server only 97 responds with a port number, and the EPRT (extended port) command, 98 which allows the client to supply either an IPv4 or an IPv6 address 99 (and a port) to the server. 101 A survey done in April of 2009 of 25 randomly picked and/or well- 102 known FTP sites reachable over IPv4 showed that only 12 of them 103 supported EPSV over IPv4. Additionally, only 2 of those 12 indicated 104 that they supported EPSV in response to the FEAT command introduced 105 in [RFC2389] that asks the server to list its supported features. 106 One supported EPSV but not FEAT. In 5 cases, issuing the EPSV 107 command to the server led to a significant delay, in 3 cases followed 108 by a control channel reset. All 25 servers were able to successfully 109 complete a transfer in traditional PASV passive mode as required by 110 [RFC1123]. More testing showed that the use of an address family 111 argument with the EPSV command is widely mis- or unimplemented in 112 servers. The additional tests with more servers showed that 113 approximately 65% of FTP servers support EPSV successfully and around 114 96% support PASV successfully. Clients were not extensively tested, 115 but previous experience from the author suggests that most clients 116 support PASV, with the notable exception of the command line client 117 included with Windows, which only supports active mode. This FTP 118 client uses the original PORT command when running over IPv4 and EPRT 119 when running over IPv6. 121 Although these issues can and should be addressed by modifying 122 clients and servers to support EPSV successfully (see 123 [I-D.liu-ftp64-extension]), such modifications may not appear widely 124 in a timely fashion. Also, network operators who may want to deploy 125 IPv6-to-IPv4 translation generally don't have control over client or 126 server implementations. As such, this document standardizes an FTP 127 Application Layer Gateway (ALG) that will allow unmodified IPv6 FTP 128 clients to interact with unmodified IPv4 FTP servers successfully 129 when using FTP for simple file transfers between a single client and 130 a single server. 132 Clients that want to engage in more complex behavior, such as server- 133 to-server transfers, may make an FTP application layer gateway (ALG) 134 go into transparent mode by issuing AUTH or ALGS commands as 135 explained in Section 5. 137 The recommendations and specifications in this document apply to all 138 forms of IPv6-to-IPv4 translation, including stateless translation 139 such as [RFC2765] or [I-D.ietf-behave-v6v4-xlate] as well as stateful 140 translation such as [I-D.ietf-behave-v6v4-xlate-stateful]. 142 2. Notational Conventions 144 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 145 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 146 document are to be interpreted as described in [RFC2119]. 148 3. Terminology 150 Within the context of this document, the words "client" and "server" 151 refer to FTP client and server implementations, respectively. An FTP 152 server is understood to be an implementation of the FTP protocol 153 running on a server system with a stable address, waiting for clients 154 to connect and issue commands that eventually start data transfers. 155 Clients interact with servers using the FTP protocol, and store 156 (upload) files to, and retrieve (download) files from one or more 157 servers. This either happens interactively under control of a user, 158 or is done as an unattended background process. Most operating 159 systems provide a web browser that implements a basic FTP client, as 160 well as a command line client. Third-party FTP clients are also 161 widely available. 163 Other terminology is derived from the documents listed in the 164 references section. Note that this document cannot be fully 165 understood on its own; it depends on background and terminology 166 outlined in the references. 168 4. ALG overview 170 The most robust way to solve an IP version mismatch between FTP 171 clients and FTP servers would be by changing clients and servers 172 rather than using an IPv6-to-IPv4 translator for the data channel and 173 using an application layer gateway on the control channel. As such, 174 it is recommended to update FTP clients and servers as required for 175 IPv6-to-IPv4 translation support where possible, to allow proper 176 operation of the FTP protocol without the need for ALGs. 178 On the other hand, network operators or even network administrators 179 within an organization often have little influence over the FTP 180 client and server implementations used over the network. For those 181 operators and administrators, deploying an ALG may be the only way to 182 provide a satisfactory customer experience. So, even though not the 183 preferred solution, this document standardizes the functionality of 184 such an ALG in order to promote consistent behavior between ALGs in 185 an effort to minimize their harmful effects. 187 Operators and administrators are encouraged to only deploy an FTP ALG 188 for IPv6-to-IPv4 translation when the FTP ALG is clearly needed. In 189 the presence of the ALG, EPSV commands that could be handled directly 190 by conforming servers are translated into PASV commands, introducing 191 additional complexity and reducing robustness. As such a "set and 192 forget" policy on ALGs is not recommended. 194 Note that the translation of EPSV through all translators and EPRT 195 through a stateless translator is relatively simple but supporting 196 translation of EPRT through a stateful translator is relatively 197 difficult, because in the latter case a translation mapping must be 198 set up for each data transfer using parameters that must be learned 199 from the client/server interaction over the control channel. This 200 needs to happen before the EPRT command can be translated into a PORT 201 command and passed on to the server. As such, an ALG used with a 202 stateful translator MUST support EPSV and MAY support EPRT. However, 203 an ALG used with a stateless translator SHOULD also support EPRT. 205 The ALG functionality is described as a function separate from the 206 IPv6-to-IPv4 translation function. However, in the case of EPRT 207 translation, the ALG and translator functions need to be tightly 208 coupled, so if EPRT translation is supported, it is assumed that the 209 ALG and IPv6-to-IPv4 translation functions are integrated within a 210 single device. 212 5. Control channel translation 214 The IPv6-to-IPv4 FTP ALG intercepts all TCP sessions towards port 21 215 for IPv6 destination addresses that map to IPv4 destinations 216 reachable through an IPv6-to-IPv4 translator. The FTP ALG implements 217 the Telnet protocol ([RFC0854]) used for control channel interactions 218 to the degree necessary to interpret commands and responses and re- 219 issue those commands and responses, modifying them as outlined below. 220 Telnet option negotiation attempts by either the client or the 221 server, except for those allowed by [RFC1123], MUST be rejected by 222 the FTP ALG without relaying those attempts. This avoids the 223 situation where the client and the server negotiate Telnet options 224 that are unimplemented by the FTP ALG. 226 There are two ways to implement the control channel ALG: 228 1. The ALG terminates the IPv6 TCP session, sets up a new IPv4 TCP 229 session towards the IPv4 FTP server, and relays commands and 230 responses back and forth between the two sessions. 232 2. Packets that are part of the control channel are translated 233 individually. 235 As they ultimately provide the same result, either implementation 236 strategy, or any other that is functionally equivalent, can be used. 238 In the second case, an implementation MUST have the ability to track 239 and update TCP sequence numbers when translating packets as well as 240 the ability to break up packets into smaller packets after 241 translation, as the control channel translation could modify the 242 length of the payload portion of the packets in question. Also, FTP 243 commands/responses or Telnet negotiations could straddle packet 244 boundaries, so in order to be able to perform the ALG function, it 245 can prove necessary to reconstitute Telnet negotiations and FTP 246 commands and responses from multiple packets. 248 If the client issues the AUTH command, then the client is attempting 249 to negotiate [RFC2228] security mechanisms which are likely to be 250 incompatible with the FTP ALG function. In this situation, the FTP 251 ALG MUST switch to transparently forwarding all data on the control 252 channel in both directions until the end of the control channel 253 session. This requirement applies regardless of the response from 254 the server. In other words, it is the fact that the client attempts 255 the AUTH negotiation that requires the ALG to become transparent, 256 whether or not the attempt is successful. The transparency 257 requirement applies to the commands and responses flowing between the 258 client and the server. It is possible that commands or responses 259 that were sent through the ALG before the AUTH command was issued 260 were changed in length so TCP sequence numbers in packets entering 261 the ALG and packets exiting the ALG no longer match. In transparent 262 mode, the ALG MUST continue to adjust sequence numbers if it was 263 doing so before entering transparent mode as the result of the AUTH 264 command. The ALGS command (Section 11) can also be used to disable 265 the ALG functionality, but the control channel must then still be 266 monitored for subsequent ALGS commands that re-enable the ALG 267 functionality. 269 There have been FTP ALGs for the purpose of making active FTP work 270 through IPv4 NATs for a long time. Another type of ALG would be one 271 that imposes restrictions required by security policies. Multiple 272 ALGs of different types can be implemented as a single entity. If 273 such a multi-purpose ALG forbids the use of the AUTH command for 274 policy reasons, the side effect of making the ALG stop performing the 275 translations described here, as well as other possible interventions 276 related to IPv6-to-IPv4 translation, MUST be retained even if the ALG 277 responds to the AUTH command with an error and does not propagate the 278 command to the server. This way, any time a client issues the AUTH 279 command, it knows that an ALG will be in transparent mode afterwards. 280 Implementers are further advised that unlike hosts behind an IPv4 281 NAT, IPv6 hosts using an IPv6-to-IPv4 translator will normally have 282 the ability to execute FTP over IPv6 without interference from the 283 IPv6-to-IPv4 translator or the ALG, so an IPv6-to-IPv4 translation 284 FTP ALG is not the best place to implement security policies. 286 [RFC2640] specifies the ability for clients and servers to negotiate 287 the language used text messages between the two of them. This will 288 typically involve the use of UTF-8 encoded characters outside the 289 7-bit ASCII range. Because client compatibility with RFC 2640 is 290 unknown, ALGs need adopt a conservative position in this regard. 291 This means that an ALG MUST NOT use any characters outside of the 292 7-bit ASCII range in the text that it generates. However, an ALG 293 MUST translate and/or forward commands and responses containing UTF-8 294 encoded text when those occur. 296 In the sections that follow, a number of well-known response numbers 297 are shown, along with the descriptive text that is associated with 298 that response number. However, this text is not part of the 299 specification of the response. As such, implementations MAY use the 300 response text shown or they MAY show a different response text for a 301 given response number. Requirements language only applies to the 302 response number. 304 6. EPSV to PASV translation 306 Although many IPv4 FTP servers support the EPSV command, some servers 307 react adversely to this command, and there is no reliable way to 308 detect in advance that this will happen. As such, an FTP ALG SHOULD 309 translate all occurrences of the EPSV command issued by the client to 310 the PASV command, and reformat a 227 response as a corresponding 229 311 response. However, an ALG MAY forego EPSV to PASV translation if it 312 has positive knowledge, either through administrative configuration 313 or learned dynamically, that EPSV will be successful without 314 translation to PASV. 316 For instance, if the client issues EPSV (or EPSV 2 to indicate IPv6 317 as the network protocol), this is translated to the PASV command. If 318 the server with address 192.0.2.31 then responds with: 320 227 Entering Passive Mode (192,0,2,31,237,19) 322 The FTP ALG reformats this as: 324 229 Entering Extended Passive Mode (|||60691|) 326 The ALG SHOULD ignore the IPv4 address in the server's 227 response. 327 This is the behavior that is exhibited by most clients and is needed 328 to work with servers that include [RFC1918] addresses in their 227 329 responses. However, if the 227 response contains an IPv4 address 330 that does not match the destination of the control channel, the FTP 331 ALG MAY send a 425 response to the client instead of the 229 332 response, e.g.: 334 425 Can't open data connection 336 It is important that the response is in the 4xx range to indicate a 337 temporary condition. 339 If the client issues an EPSV command with a numeric argument other 340 than 2, the ALG MUST NOT pass the command on to the server, but 341 rather respond with a 522 error, e.g.: 343 522 Network protocol not supported 345 If the client issues EPSV ALL, the FTP ALG MUST NOT pass this command 346 to the server, but respond with a 504 error, e.g.: 348 504 Command not implemented for that parameter 350 This avoids the situation where an FTP server reacts adversely to 351 receiving a PASV command after the client used the EPSV ALL command 352 to indicate that it will only use EPSV during this session. 354 7. EPRT to PORT translation 356 Should the IPv6 client issue an EPRT command, the FTP ALG MAY 357 translate this EPRT command to a PORT command. The translation is 358 different depending on whether the translator is a stateless one-to- 359 one translator or a stateful one-to-many translator. 361 7.1. Stateless EPRT translation 363 If the address specified in the EPRT command is the IPv6 address used 364 by the client for the control channel session, then the FTP ALG 365 reformats the EPRT command into a PORT command with the IPv4 address 366 that maps to the client's IPv6 address. The port number MUST be 367 preserved for compatibility with stateless translators. For 368 instance, if the client with IPv6 address 2001:db8:2::31 issues the 369 following EPRT command: 371 EPRT |2|2001:db8:2::31|5282| 373 Assuming the IPv4 address that goes with 2001:db8:2::31 is 374 192.0.2.31, the FTP ALG reformats this as: 376 PORT 192,0,2,31,20,162 378 If the address specified in the EPRT command is an IPv4 address or an 379 IPv6 address that is not the IPv6 address used by the client for the 380 control session, the ALG SHOULD NOT attempt any translation, but pass 381 along the command unchanged. 383 7.2. Stateful EPRT translation 385 If the address in the EPRT command is the IPv6 address used by the 386 client for the control channel, the stateful translator selects an 387 unused port number in combination with the IPv4 address used for the 388 control channel towards the FTP server, and sets up a mapping from 389 that transport address to the one specified by the client in the EPRT 390 command. The PORT command with the IPv4 address and port used on the 391 IPv4 side of the mapping is only issued towards the server once the 392 mapping is created. Initially, the mapping is such that either any 393 transport address or the FTP server's IPv4 address with any port 394 number is accepted as a source, but once the three-way handshake is 395 complete, the mapping SHOULD be narrowed to only match the negotiated 396 TCP session. 398 If the address specified in the EPRT command is an IPv4 address or an 399 IPv6 address that is not the IPv6 address used by the client for the 400 control session, the ALG SHOULD NOT attempt any translation, but pass 401 along the command unchanged. 403 If the client with IPv6 address 2001:db8:2::31 issues the EPRT 404 command: 406 EPRT |2|2001:db8:2::31|5282| 408 And the stateful translator uses the address 192.0.2.31 on its IPv4 409 interface, a mapping with destination address 192.0.2.31 and 410 destination port 60192 towards 2001:db8:2::31 port 5282 may be 411 created, after which the FTP ALG reformats the EPRT command as: 413 PORT 192,0,2,31,235,32 415 8. Default port 20 translation 417 If the client does not issue an EPSV/PASV or EPRT/PORT command prior 418 to initiating a file transfer, it is invoking the default active FTP 419 behavior where the server sets up a TCP session towards the client. 420 In this situation, the source port number is the default FTP data 421 port (port 20) and the destination port is the port the client uses 422 as the source port for the control channel session. 424 In the case of a stateless translator, this does not pose any 425 problems. In the case of a stateful translator, the translator MAY 426 accept incoming connection requests from the server on the IPv4 side 427 if the transport addresses match that of an existing FTP control 428 channel session, with the exception that the control channel session 429 uses port 21 and the new session port 20. In this case, a mapping is 430 set up towards the same transport address on the IPv6 side that is 431 used for the matching FTP control channel session. 433 An ALG/translator MAY monitor the progress of FTP control channels 434 and only attempt to perform a mapping when an FTP client has started 435 a file transfer without issuing the EPSV, PASV, EPRT or PORT 436 commands. 438 9. Both PORT and PASV 440 [RFC0959] allows a client to issue both PORT and PASV to use non- 441 default ports on both sides of the connection. However, this is 442 incompatible with the notion that with PASV, the data connection is 443 made from the client to the server, while PORT reaffirms the default 444 behavior where the server connects to the client. As such, the 445 behavior of an ALG is undefined when a client issues both PASV and 446 PORT. Implementations SHOULD NOT try to detect the situation where 447 both PASV and PORT commands are issued prior to a command that 448 initiates a transfer, but rather, apply the same translation they 449 would have if there had not been a PASV command prior to a PORT 450 command or a PORT command prior to a PASV command. 452 10. Default behavior 454 Whenever the client issues a command which the ALG is not set up to 455 translate, either because the command is not specified in this 456 document, the command is not part of any FTP specification, the ALG 457 functionality is disabled administratively for the command in 458 question, or translation does not apply for any other reason, the 459 command MUST be passed on to the server without modification, and the 460 server response MUST be passed on to the client without modification. 462 For example, if the client issues the PASV command, this command is 463 passed on to the server transparently, and the server's response to 464 the client. 466 11. The ALGS command 468 ALGs SHOULD support the new ALGS (ALG status) command that allows 469 clients to query and set the ALG's status. Note that this command 470 MUST NOT be implemented in FTP servers. If those recognize the 471 command, the best course of action would be to return a 202 response: 473 202 Command not implemented, superfluous at this site 475 However, as FTP servers don't implement the command, there is no 476 reason for them to specifically recognize this command, and returning 477 any 50x response that is normally returned when commands are not 478 recognized is appropriate. A client can use the ALGS command to 479 request the ALG's status and to enable and disable EPSV to PASV and, 480 if implemented, EPRT to PORT translation. There are three possible 481 arguments to the ALGS command: 483 ALGS STATUS64 484 The ALG is requested to return the EPSV and EPRT 485 translation status. 487 ALGS ENABLE64 488 The ALG is requested to enable translation. 490 ALGS DISABLE64 491 The ALG is requested to disable translation. 493 The ALG SHOULD enable or disable translation as requested. If EPRT 494 to PORT translation is supported, ALGS ENABLE64 enables it and ALGS 495 DISABLE64 disables it along with enabling or disabling EPSV to PASV 496 translation, respectively. If EPRT to PORT translation is not 497 supported, ALGS ENABLE64 only enables EPSV to PASV translation. 498 After an ALGS command with any of the three supported arguments, the 499 ALG returns a 216 response indicating the type of translation that 500 will be performed. There are four possible keywords that follow the 501 216 response code: 503 216 NONE 504 Neither EPSV nor EPRT translation is performed. 506 216 EPSV 507 EPSV is translated to PASV, no EPRT translation is 508 performed. 510 216 EPRT 511 EPRT is translated to PORT, no EPSV translation is 512 performed. 514 216 EPSVEPRT 515 EPSV is translated to PASV, EPRT is translated to 516 PORT. 518 The translation type MAY be followed by a space and additional 519 descriptive text until end-of-line. Failure to set the requested 520 translation mode is not an error condition, and is thus indicated by 521 the keyword that shows the translation status of the ALG following 522 the 216 response, and not with an error code response. 524 If the ALGS command is not implemented, the command SHOULD be passed 525 on to the server without modification. If there is no argument to 526 the ALGS command, or the argument is not one of STATUS64, ENABLE64 or 527 DISABLE64 (or an argument specified by a supported newer document), a 528 504 or 502 error SHOULD be returned. 530 The Augmented Backus-Naur Form (ABNF) notation (see [RFC5234]) of the 531 ALGS command and its response are as follows: 533 algs-command = "ALGS" SP algs-token CRLF 534 algs-token = "STATUS64" / "ENABLE64" / "DISABLE64" 536 algs-response = (ok-response / error-response) CRLF 537 ok-response = "216" SP response-token [ freetext ] 538 response-token = "NONE" / "EPSV" / "EPRT" / "EPSVEPRT" 539 error-response = not-implemented / invalid-parameter 540 not-implemented = "502" [ freetext ] 541 invalid-parameter = "504" [ freetext ] 542 freetext = (SP *VCHAR) 544 12. Timeouts and translating to NOOP 546 Wherever possible, control channels SHOULD NOT time out while there 547 is an active data channel. A timeout of at least 30 seconds is 548 RECOMMENDED for data channel mappings created by the FTP ALG that are 549 waiting for initial packets. 551 Whenever a command from the client is not propagated to the server, 552 the FTP ALG instead issues a NOOP command in order to keep the 553 keepalive state between the client and the server synchronized. The 554 response to the NOOP command MUST NOT be relayed back to the client. 555 An implementation MAY wait for the server to return the 200 response 556 to the NOOP command and translate that 200 response into the response 557 the ALG is required to return to the client. This way, the ALG never 558 has to create new packets to send to the client, but it can limit 559 itself to modifying packets transmitted by the server. If the server 560 responds with something other than 200 to the NOOP command, the ALG 561 SHOULD tear down the control channel session and log an error. 563 13. IANA considerations 565 IANA is requested to add to the FTP Commands and Extensions registry 566 the following entry: 568 Command Name 569 ALGS 571 FEAT Code 572 -N/A- 574 Description 575 FTP64 ALG status 577 Command Type 578 -N/A- 580 Conformance Requirements 581 o 583 Reference 584 RFC TBD Section 11 586 [TO BE REMOVED: This registration should take place at the following 587 location: http://www.iana.org/assignments/ftp-commands-extensions/ 588 ftp-commands-extensions.xhtml] 590 14. Security considerations 592 In the majority of cases, FTP is used without further security 593 mechanisms. This allows an attacker with passive interception 594 capabilities to obtain the login credentials, and an attacker that 595 can modify packets to change the data transferred. However, FTP can 596 be used with TLS in order to solve these issues. IPv6-to-IPv4 597 translation and the FTP ALG do not impact the security issues in the 598 former case nor the use of TLS in the latter case. However, if FTP 599 is used with TLS or another authentication mechanism, the ALG 600 function is not performed so only passive transfers from a server 601 that implements EPSV or a client that supports PASV will succeed. 603 For general FTP security considerations, see [RFC2577]. 605 15. Contributors 607 Dan Wing, Kentaro Ebisawa, Remi Denis-Courmont, Mayuresh Bakshi, 608 Sarat Kamisetty, Reinaldo Penno, Alun Jones, Dave Thaler, Mohammed 609 Boucadair, Mikael Abrahamsson, Dapeng Liu, Michael Liu, Andrew 610 Sullivan, Anthony Bryan and Ed Jankiewicz contributed ideas and 611 comments. Dan Wing ran experiments with a large number of FTP 612 servers that were very illuminating; many of the choices underlying 613 this document are based on his results. 615 16. Acknowledgements 617 Iljitsch van Beijnum is partly funded by Trilogy, a research project 618 supported by the European Commission under its Seventh Framework 619 Program. 621 17. References 623 17.1. Normative References 625 [RFC0854] Postel, J. and J. Reynolds, "Telnet Protocol 626 Specification", STD 8, RFC 854, May 1983. 628 [RFC0959] Postel, J. and J. Reynolds, "File Transfer Protocol", 629 STD 9, RFC 959, October 1985. 631 [RFC1123] Braden, R., "Requirements for Internet Hosts - Application 632 and Support", STD 3, RFC 1123, October 1989. 634 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 635 Requirement Levels", BCP 14, RFC 2119, March 1997. 637 [RFC2228] Horowitz, M., "FTP Security Extensions", RFC 2228, 638 October 1997. 640 [RFC2428] Allman, M., Ostermann, S., and C. Metz, "FTP Extensions 641 for IPv6 and NATs", RFC 2428, September 1998. 643 [RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax 644 Specifications: ABNF", STD 68, RFC 5234, January 2008. 646 17.2. Informative References 648 [RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and 649 E. Lear, "Address Allocation for Private Internets", 650 BCP 5, RFC 1918, February 1996. 652 [RFC2389] Hethmon, P. and R. Elz, "Feature negotiation mechanism for 653 the File Transfer Protocol", RFC 2389, August 1998. 655 [RFC2577] Allman, M. and S. Ostermann, "FTP Security 656 Considerations", RFC 2577, May 1999. 658 [RFC2640] Curtin, B., "Internationalization of the File Transfer 659 Protocol", RFC 2640, July 1999. 661 [RFC2765] Nordmark, E., "Stateless IP/ICMP Translation Algorithm 662 (SIIT)", RFC 2765, February 2000. 664 [I-D.ietf-behave-v6v4-xlate-stateful] 665 Bagnulo, M., Matthews, P., and I. Beijnum, "Stateful 666 NAT64: Network Address and Protocol Translation from IPv6 667 Clients to IPv4 Servers", 668 draft-ietf-behave-v6v4-xlate-stateful-12 (work in 669 progress), July 2010. 671 [I-D.ietf-behave-v6v4-xlate] 672 Li, X., Bao, C., and F. Baker, "IP/ICMP Translation 673 Algorithm", draft-ietf-behave-v6v4-xlate-05 (work in 674 progress), December 2009. 676 [I-D.liu-ftp64-extension] 677 Liu, D., Beijnum, I., and Z. Cao, "FTP extension for IPv4/ 678 IPv6 transition", draft-liu-ftp64-extension-00 (work in 679 progress), October 2010. 681 [Bernstein] 682 Bernstein, D., "PASV security and PORT security", 2000, 683 . 685 Author's Address 687 Iljitsch van Beijnum 688 IMDEA Networks 689 Avda. del Mar Mediterraneo, 22 690 Leganes, Madrid 28918 691 Spain 693 Email: iljitsch@muada.com